CCAMLR Science, Volume 7 (2000)
Commercial exploitation of finfish in the southern Scotia Arc took place from 1977/78 to 1989/90, and was in its heyday from 1977/78 to 1981/82. Except for Elephant Island, the state of fish stocks of the southern Scotia Arc region has been accorded little attention until 1998, despite substantial catches in the first four years of the fishery and ample opportunity to sample these catches. The only scientific surveys of these stocks during these years were conducted by Germany in 1985, and by Spain in 1987 and 1991. More recently, the US Antarctic Marine Living Resources (US AMLR) Program carried out two extensive surveys around Elephant Island and the lower South Shetland Islands in March 1998 and around the South Orkney Islands in March 1999. In this paper, the authors present new data on species composition, species groups, length compositions, length-weight relationships, length at sexual maturity and length at first spawning, gonadosomatic indices and oocyte diameter. Lesser Antarctic or peri-Antarctic species predominated in the fish fauna. Species groups differed by up to 55-60% from one shelf area to the other, mostly due to differences in the abundance of the predominant species on each shelf area and the increase in the number of high-Antarctic species in the South Orkney Islands. Length compositions and the proportion of large (= old) specimens in the populations provided no evidence of illegal fishing since the closure of the region some 10 years ago. Differences in length-weight relationships between areas were primarily due to differences in length compositions of the fish caught, but did not suggest geographical differences in length-to-weight growth. Differences in estimates of length at sexual maturity and length at first spawning indicate that in some species final maturation of the gonads takes one year or more, whereas in others gonad maturation was completed within one season. Length at sexual maturity and length at first spawning in Champsocephalus gunnari was one year later on the southerly grounds than at South Georgia. The distribution of gonadosomatic indices in March suggested that Lepidonontothen squamifrons was spawning while Trematomus hansoni was already coming to the end of its reproductive season. Chionodraco rastropinosus and Trematomus eulepiodus were close to spawning. Other species, such as Notothenia rossii and Pseudochaenichtyhys georgianus, spawn at least one to two months later than at South Georgia. Channichthyids (except C. gunnari), Notothenia rossii and Notothenia coriiceps have egg diameters of 4.3-5.2 mm at spawning. The Trematomus species investigated have egg diameters of 2.8-3.2 mm while in species of the genera Gobionotothen and Lepidonotothen, egg diameters rarely exceeded 2.0 mm. Measurements of oocyte diameters confirmed the findings on spawning time estimated from gonadosomatic indices. |
Monitoring of demersal fish at inshore sites of the South Shetland Islands has continued at Potter Cove from 1991 to 1999, and at Harmony Cove, Nelson Island, in the austral summer of 1995/96. Still evident is the decline in trammel-net catches of fjord-dwelling Notothenia rossii and Gobionotothen gibberifrons in relation to the non-commercially fished Notothenia coriiceps, which was previously reported for the period 1983-1990. A trend of increasing N. rossii catches was observed, but the levels of relative abundance of this species and G. gibberifrons are well below those found in the early 1980s. These results are supported by our knowledge about the diet of the piscivorous Antarctic shag (Phalacrocorax bransfieldensis) in the South Shetland-Antarctic Peninsula area in this decade. The decrease in recruitment to the inshore sub-populations of N. rossii and G. gibberifrons over the last 16 years was most probably caused by the offshore commercial fishery in the area in the late 1970s. This interpretation is consistent with historical information on offshore commercial fishing and with the results of scientific surveys in the area. |
Stocks of finfish exploited around the South Orkney Islands (Subarea 48.2) suffered substantial declines from split-year 1977/78 to 1989/90. The biomass of several species of finfish has been monitored through scientific bottom trawl surveys within the 500 m isobath of the South Orkney Islands, most recently by the Federal Republic of Germany in 1985, Spain in 1991 and the USA in 1999. From these surveys, estimates of total stock biomass were computed for the eight species of finfish which were most abundant in catches. Biomass levels and size composition from the March 1999 trawl survey were compared to previous surveys conducted in February 1985 and January to February 1991. Species examined were Gobionotothen gibberifrons, Lepidonotothen squamifrons, Pseudochaenichthys georgianus, Champsocephalus gunnari, Chaenocephalus aceratus, Chionodraco rastrospinosus, Notothenia rossii and Lepidonotothen larseni. Despite substantial variability in point estimates, biomass levels of most species appear to be unchanged or may have declined slightly since 1991. The stock of C. gunnarii is currently extremely low, while there appear to be strong signs of recovery for N. rossii. However, overall levels of biomass indicate very little potential for commercial exploitation at this time. |
The need to protect young fish and spawning aggregations in fisheries generally, and specifically in the Champsocephalus gunnari fishery in Subarea 48.3, is reviewed. Mechanisms available to achieve these objectives are discussed, including the measures put in place by CCAMLR to date. These measures are re-examined in the light of new information, and a strategy for the future protection of young fish and spawning aggregations of C. gunnari in Subarea 48.3 is considered. This comprises a combination of existing measures, including minimum mesh size and fish size, and a proposed new closed season from 1 March to 31 May. |
The total circumpolar abundance of Antarctic krill (Euphausia superba) is estimated using recent measurements of krill density from acoustic surveys and historical information on the overall range of krill. The biomass estimates fall between 60 and 155 million tonnes - at the low end of values that have been suggested in the past. The differences between our estimates and others can be explained by a number of factors such as: an underestimation of the range or the acoustic biomass estimates; the possibility of a large, undetected krill population; and the overestimation of the demand for krill by predators. Even if these low krill biomass estimates are correct, regional and total precautionary limits are still likely to rise as a result of new surveys because the method used to calculate precautionary limits uses a value of 11% of the estimated pre-exploitation biomass. Additionally, the current precautionary catch limits in the South Atlantic are set using a superseded acoustic target strength which has effectively underestimated the krill biomass by a factor of three, so the new survey of the South Atlantic in January 2000 may result in an effective biomass which is greater than the value used in the past. The seasonal and local consequences of large catch limits will have to be taken into account when managing an expanded krill fishery, and the appropriateness of using the existing statistical divisions as management areas will have to be considered. |
A potential method is presented for combining data collected as part of the CCAMLR ecosystem monitoring program (CEMP) into a single index for each of predator, prey and environmental parameters. The paper is divided into four main parts. The first part develops the proposed method of forming summary indices, which is based on the usual theory of multivariate statistics and takes into account the covariance between parameters. The second part reports on a Monte Carlo simulation study that examines the robustness of the indices to missing data and the degree of correlation between parameters. These trials show that missing values were unlikely to be a problem for time series of parameters that are highly correlated (>0.6). Criteria for inclusion of parameters in the indices are discussed when parameters are moderately or poorly correlated. The third part uses further simulation tests to examine the power of the statistical procedure adopted by WG-EMM in 1996 for identifying anomalies in CEMP parameters. The power of the procedure to detect anomalies was found to fall to low levels once more than a few anomalous values have appeared in the data. An alternative procedure, using estimates of the mean and variance of baseline time series, was found to have consistently better statistical power regardless of the accumulation of anomalies. The last section outlines an approach to the further development of CEMP indices for application in CCAMLR. |
A series of experiments was carried out in February 1999 at South Georgia (Subarea 48.3) to examine the effects of different weighting regimes on the incidental mortality of birds caught on longlines fishing for toothfish (Dissostichus spp.). Three regimes were examined, with weights of 4.25, 8.5 and 12.75 kg attached at 40 m intervals on a Spanish-rigged longline. There was a significant reduction in bird mortality when 8.5 kg was used compared to 4.25 kg, but no further significant reduction when 12.75 kg was used. Therefore to minimise seabird by-catch, line-weighting regimes of at least 8.5 kg at 40 m intervals should be used. Results suggested that seabird by-catch on lines using effective weighting regimes may be even further reduced where all lines being set in the vicinity use effective line-weighting regimes. The importance of a good design in experiments of this type is emphasised, because even though conditions may be held as constant as possible within experiments, the ability to separate treatment effects from other sources of variation (e.g. environmental factors) must be preserved. |
An experiment was conducted on autoline and Spanish-system longline vessels to derive a sink rate and line-weighting regime that would minimise the capture of albatrosses based on knowledge of line sink rates and albatross diving abilities. Sink rates of lines deployed into propeller turbulence, which tended to keep lines aloft, varied as a function of distance between line weights. Asymptotic sink rates (0.1-0.15 m/s) were achieved with 70 m intervals between 6.5 kg weights. Sink rates to 4 m depth were greatest with 35 m (0.44 m/s) and 50 m (0.33 m/s) between weights. For vessels using bird-scaring lines and setting lines in propeller turbulence, longline sink rates >0.3 m/s should greatly reduce the incidental take of albatrosses. For autoline vessels with gear and line-setting characteristics similar to the experimental vessel, this sink rate should be achievable with 4 kg weights distributed every 40 m on longlines. |
This paper describes the diet and foraging behaviour of Adelie penguins (Pygoscelis adeliae) at Béchervaise Island during 1998/99, a season of low breeding success. Fifty-six percent of nests with eggs failed during the three weeks following first hatch. A breeding success of 0.43 chicks cr6ched per nest was achieved, compared to previous seasons with higher annual breeding success rates ranging from 0.69 to 1.06 chicks créched per nest. Evidence from analyses of foraging location, foraging trip duration and diet suggests that the death of chicks during the guard stage was due to an inadequate rate of food supply. In most previous seasons adults have foraged both at the continental shelf edge (particularly females) and locally (particularly males). This season male penguins carried out fewer local trips and both sexes spent longer at sea than in years of higher breeding success. Meal masses brought back to the chicks were within the normal range but extended foraging trip durations reduced feeding frequency. These findings contrast with observations made in 1994/95 (a season in which all chicks died of starvation), when smaller meals were delivered and birds foraged further offshore than in this or any other season studied. The significance to CCAMLR of these variations in foraging behaviour is discussed. |
This paper provides an assessment of the distribution and abundance of Macrourus carinatus on BANZARE Bank in the southern Indian Ocean (Divisions 58.4.1 and 58.4.3) based on results from a trawl survey in 1999. This is the first quantitative assessment of M. carinatus in this region. M. carinatus was the most common species caught in each haul, comprising on average 52 ± 4% of the total biomass. The mean density of M. carinatus on BANZARE Bank was 176 ± 14 kg/km2, while the biomass was estimated to be 16 658 ± 1 325 tonnes. M. carinatus appeared to be uniformly distributed over the bank. |
Hewitt, R (2000), An index of per capita recruitment An index of per data recruitment (PCR) is proposed such that
where R1 is the proportion of age-l animals sampled in year y and M is the post-recruit mortality rate. The intent of the index is to facilitate investigation of reproductive success and the factors postulated to affect it. The formulation of PCR is based on the assumptions that: (i) post-recruit mortality does not vary over age or between years; (ii) 100% of age-l animals spawn; (iii) a representative sample of the population is available; and (iv) the proportion of age-l animals in the sample can be determined unambiguously. Normal, lognormal and uniform probability distributions of RI, and three levels of M were assumed in order to investigate the resulting distributions of PCR. Distributions of PCR are skewed toward higher values such that the dynamic range of PCR is largest with high values of R1; increasing M tends to offset this effect but only slightly. A simple population model was then constructed to test the sensitivity of PCR to relaxation of its underlying assumptions. PCR is not biased relative to recruits per spawner when mortality is constant over all ages and years, and when all age-l animals spawn. These conclusions are insensitive to changes in the shape of the functional relationship between spawners and recruits. PCR is biased low with age-specific decline in mortality and reduction in the proportion of age-l spawners. Introducing year-to-year random variability in both mortality and proportion of age-l spawners resulted in broader distributions of PCR relative to recruits per spawner but did not appear to introduce additional bias. On average, PCR will underestimate recruits per spawner by 30% if reasonable assumptions are made regarding the variability of mortality and the proportion of age-l spawners. The effectiveness of PCR to track changes in recruits per spawner over time was confirmed by introducing cycles in the shape of the functional relationship between spawners and recruits. PCR was also able to track cycles in recruits per spawner after a 20% random sampling error was added to the value of R1 used in the calculation of PCR and year-to-year random variability in mortality and proportion of age-l spawners was introduced, although errors were larger. A time series of PCR for Antarctic krill (Euphausia superba) sampled in the vicinity of the South Shetland Islands from 1979 to 1998 is presented. |
A revised bathymetric map of the South Orkney Islands (Subarea 48.2) was generated using several integrated bathymetric databases and newly available acoustic seafloor data. The region extends between 60.2o-62.2oS latitude and 42.5o-47.5oW longitude. From the integrated datasets, areas of seabed within the 500 m isobath (i.e. 0-500 m) were computed for 50 m depth intervals. Areas were calculated based on interpolated surface area of seabed incorporating seafloor slope. These results were compared to the previously reported estimates of Everson (1987) for the 50-150, 150-250 and 250-500 m depth intervals. The updated estimates are about 1 424 (20%) n miles larger in area within the 50-500 m isobaths than Everson's estimates, though changes in area are specific to the depth interval. There is a corresponding change in estimated biomass within strata when these areas are incorporated into swept area trawl survey models, though not for total estimated biomass. Of the nine species examined, the point estimate of total biomass increased from 5 to 30% for eight species and decreased by 20% for one. |
